Image blending systems may combine image intensification (II) with thermal sensing. When such systems use more than one aperture with separated optical axes, parallax compensation may be needed.
FIG. 1 shows an exemplary prior art dual aperture optical system 100, with a first aperture 110 and a second aperture 120 directed to a target object 101. The optical system 100 routes a first image 111 from the first aperture 110 through a blending device 150, such as a prism, to an eyepiece 160. Similarly, the optical system 100 routes a second image 121 from the second aperture 120 through the blending device 150 to the eyepiece 160 via a reflector 170. Due to the different perspectives of the first aperture 110 and the second aperture 120, the position of the image of the target 101 within the first aperture image 111 is different from the position of the image of the target 101 within the second aperture image 121. When the first aperture image 111 and the second aperture image 121 are blended by the blending device 150 to form a blended image 161 at the eyepiece 160, the blended image 161 displays parallax errors.
The position of the first image 111 and/or the second image 121 may be corrected so that the images of the target 101 align in the blended image 161. However, this positional correction will only be appropriate for targets at a predetermined distance from the apertures 110, 120. In particular, when the longitudinal axis of the first aperture 110 and the longitudinal axis of the second aperture 120 are aligned such that the blended images of a target at a predetermined distance are aligned in the eyepiece 160, objects at distances other than the predetermined distance are offset in the eyepiece 160. This offset increases as the target distance varies from the predetermined distance. This parallax offset may be corrected by adjusting the position of one image or the other in the eyepiece 160. For example, circuitry may be employed to adjust the position of one image or the other in the eyepiece, such that the images remain aligned even as the distance of the target 101 from the image system 100 changes. Such circuitry is described, for example, by U.S. Patent Publication No. 2007/0235634.
This type of parallax compensation generally uses the distance of the target and the distance between apertures to calculate the image offset correction amount. The distance of the target is determined from the focus setting required to produce a focused image of the target. This requires that the depth of focus of the optics is sufficiently small that a change in object distance causes the object to go out of focus, thereby requiring a change to the focus setting, before a parallax error becomes apparent. However, under certain conditions, such as a large change of temperature, the focus setting may not be a function of the distance of the target alone. Optics with a sufficiently small depth of focus are typically sensitive to thermal defocus, thereby requiring a change to the focus setting when subjected to a large change of temperature. Therefore, the correct image offset for parallax compensation cannot be determined from the focus setting alone under these conditions. There is a need in the industry to address the abovementioned deficiencies.